Importance of Ludwigia grandiflora as invasive weed on meadows and pastures in Western France

Abstract : In flooded meadows, Large Water Primrose (Ludwigia grandiflora ssp. hexapetala) has become an invasive weed since years in many Western marshes. Settlement occurs both by cuttings and seedlings. Our purpose was (i) to assess progressive colonization in three sites and (ii) to show adaptive traits of Ludwigia "terrestrial forms" using another site with a strong moisture gradient, and (iii) to get recommendations for managers. (i) In the Natural Regional Park of Brière, colonization occurred through dykes. The first appearance was probably due to a dirty dredging engine. In Mazerolles polder, it was due to flooding and was increased by pumping in colonized dykes with much fragmentation. In Apigné meadow bad drainage led to colonization by Ludwigia. Field maps assess progressive colonization on many areas. (ii) Adaptation to land conditions led to reduced biomass of the whole plant and increased ratio between roots and stems. Cumulative stem length was only 21 cm in dry conditions, while it reached 2064 cm in aquatic forms. After settlement, plants are able to survive for years provided there is some remaining moisture in the soil. (iii) As a consequence for managers, barriers limiting dispersion of cuttings should be established in dykes but also along the hydraulic network leaving reed strips. Flooding events should lead to careful field survey to pick up the cuttings. The main problem is when fertile seeds are dispersed all over the area as in Mazerolles. No prevention exists, thus attempts of destroying Ludwigia mats are experimented.